FIG. 4 shows an example of a high frequency apparatus of the prior art. That is, the conventional high frequency apparatus 100 comprises input terminal 2 in connection to antenna 1, and input filter 3 connected to input terminal 2. Apparatus 100 also comprises AGC amplifier 4 connected to an output of input filter 3 and provided with an AGC control terminal, tuning filter 5 connected to an output of AGC amplifier 4, mixer 6 having one of input ports connected with an output of tuning filter 5, and intermediate frequency filter 7 connected to an output of mixer 6. Apparatus 100 further comprises intermediate frequency amplifier 8 connected to an output of intermediate frequency filter 7, and demodulator 9 connected to an output of intermediate frequency amplifier 8.
Furthermore, apparatus 100 comprises error correction circuit 10 connected to an output of demodulator 9, output terminal 11 connected to an output of error correction circuit 10, AGC voltage detector circuit 12 connected between an output of mixer 6 and the AGC control terminal of AGC amplifier 4, and local oscillator 13 connected to the other input port of mixer 6.
Moreover, apparatus 100 comprises PLL circuit 14 connected into a loop circuit with local oscillator 13, an output side of PLL circuit 14 connected to a tuning input terminal of tuning filter 5, and crystal resonator 15 connected to another input port of PLL circuit 14.
Cellular telephone 16 equipped with antenna 16a is installed in high frequency apparatus 100. Error correction circuit 10 comprises a serially connected unit of Viterbi decoding circuit 10a and Reed-Solomon encoding circuit 10b. 
High frequency apparatus 100 constructed as above operates in a manner which is described hereinafter. High frequency digital signals input to antenna 1 are fed to tuning filter 5 where a desired wave to be received is tuned and selected. The selected wave is mixed with an output of local oscillator 13 by mixer 6, and an intermediate frequency wave is output from the output side of intermediate frequency amplifier 8. This intermediate frequency wave is demodulated by demodulator 9, and output from output terminal 11 after subjected to error correction by error correction circuit 10.
The prior art technique pertaining to this invention is disclosed, for example, in Japanese Patent Unexamined Publication, No. 2000-13357.
However, when conversation is made by using cellular telephone 16 installed in such conventional high frequency apparatus 100, the communication signal of cellular telephone 16 is transmitted from antenna 16a. When this occurs, the communication signal output from cellular telephone 16 is received in antenna 1. Therefore, if the signal received in antenna 1 has a frequency adjacent to the frequency band of the desired wave being received, the communication signal (i.e., the transmission signal) output from cellular telephone 16 becomes an interfering signal, and this gives rise to a problem that it makes difficult for apparatus 100 to receive the originally desired signals.
Accordingly, the present invention is to provide a high frequency apparatus that eliminates the problem attributable to undesirable interfering signals such as the above.